Separator for electrochemical cell and method for its manufacture

ABSTRACT

An electrode/separator assembly for use in an electrochemical cell includes a current collector; a porous composite electrode layer adhered to the current collector, said electrode layer comprising at least electroactive particles and a binder; and a porous composite separator layer comprising inorganic particles substantially uniformly distributed in a polymer matrix to form nanopores and having a pore volume fraction of at least 25%, wherein the separator layer is secured to the electrode layer by a solvent weld at the interface between the two layers, said weld comprising a mixture of the binder and the polymer. Methods of making and using the assembly are also described.

CROSS-REFERENCE

The present application is a divisional application of U.S. patentapplication Ser. No. 14/227,588, entitled “SEPARATOR FOR ELECTROCHEMICALCELL AND METHOD FOR ITS MANUFACTURE,” filed on Mar. 27, 2014. U.S.patent application Ser. No. 14/227,588 claims priority to U.S. patentapplication Ser. No. 12/196,203, entitled “SEPARATOR FOR ELECTROCHEMICALCELL AND METHOD FOR ITS MANUFACTURE,” filed on Aug. 21, 2008, now U.S.Pat. No. 8,697,273. U.S. patent application Ser. No. 12/196,203 claimspriority to U.S. Provisional Application No. 60/957,101, entitled“SEPARATOR FOR ELECTROCHEMICAL CELL AND METHOD FOR ITS MANUFACTURE,”filed on Aug. 21, 2007. The entire contents of each of theabove-referenced applications are hereby incorporated by reference intheir entirety for all purposes.

COPYRIGHT NOTICE

This patent disclosure may contain material that is subject to copyrightprotection. The copyright owner has no objection to the facsimilereproduction by anyone of the patent document or the patent disclosureas it appears in the U.S. Patent and Trademark Office patent file orrecords, but otherwise reserves any and all copyright rights.

INCORPORATION BY REFERENCE

All patents, patent applications and publications cited herein arehereby incorporated by reference in their entirety in order to morefully describe the state of the art as known to those skilled therein asof the date of the invention described herein.

FIELD OF THE INVENTION

This invention relates generally to electrochemical cells. Morespecifically, the invention relates to battery cells. Most specifically,the invention relates to separator membranes for electrochemical batterycells

BACKGROUND OF THE INVENTION

Separator membranes are important components of batteries. Thesemembranes serve to prevent contact of the anode and cathode of thebattery while permitting electrolyte to pass there through.Additionally, battery performance attributes such as cycle life andpower can be significantly affected by the choice of separator. Safetycan also be related to separator attributes, and certain separators areknown to reduce occurrence of Li metal plating at the anode and evendendrite formation.

Separator membranes of battery cells are, in some instances, formed frombodies of porous polymer materials. In other instances, separatormembranes are formed from bodies of fibrous or particulate material, andsuch materials can include glass fibers, mineral fibers such asasbestos, ceramics, synthetic polymeric fibers as well as naturalpolymeric fibers such as cellulose.

There are a number of problems with the presently utilized separatormembranes. Such membranes materials are often expensive, and given thefact that a typical battery system will include relatively large volumesof membranes, the cost of the membranes can be a significant componentof overall battery costs. Typical separator membranes used in prior artlithium ion cells are made from polymers such as polyethylene orpolypropylene, and they may be fabricated in either a wet or a dryprocess. In a wet process, polymers of differing molecular weights andan oil are first blended, and then melt extruded to yield a film. Thefilm is subsequently subjected to an extraction (“wet”) step, in whichthe oil/low molecular weight polyolefins are extracted from the highermolecular weight solid film to leave a porous film. In the dry processfor a three layer film, separate layers of polymer film are laminated,drawn down, and annealed so as to provide a polymer structure which hasoriented crystallites. The sheet is then rapidly uniaxially stretched toobtain porosity. A similar process is used for dry processing of singlelayer films. These processes are relatively expensive, and membranesproduced thereby have costs in the range of several dollars per squaremeter. The high cost of the separators translates to high cost forfinished cells. Any reduction in the cost of the membrane will translateto significant savings in the overall cost of batteries. In addition,polymer separators must maintain their size and shape as temperaturesare increased beyond the usual operating temperatures, to assurecontinued physical separation between anode and cathode. Many separatorsshrink unacceptably at increased temperatures and unacceptably allow thetwo electrodes to contact each other and thereby causing the cell torapidly discharge, further contributing to unsafe increases in celltemperature. It is an important safety feature for the separators tomaintain shape and original size and to avoid electrode contact at hightemperatures.

Inorganic composite materials have also been used as separators. Suchcomposite separators include a silica (or other ceramic) filler materialand a polymer binder. The filler and binder are blended and extruded toform a composite sheet and any volatile components are removed byextraction or evaporation to form a porous body. Other examples blendthe filler and binder to form a mixture that is applied to a substrateby various coating means, such as doctor blading, roll coating orscreen, stencil printing or gravure. In many cases, the compositeseparator materials contain a very high content of inorganic filler. Insome instances, the separators exhibit poor properties, such asmechanical properties.

Low cost battery separator membrane materials can be inefficient inpreventing dendrite bridging, and hence must be made relatively thick.However, this thickness increases the internal resistance of the batterythereby decreasing its efficiency, and also increases battery size. Inaddition, various separator materials are fragile, and this fragilitycan complicate the manufacture of battery systems and both increase costto manufacture a cell and potentially compromise safety.

Thus, there is a need for separator membranes which are efficient, lowin cost, safe and easy to utilize.

SUMMARY

A separator for electrochemical cells is described. This separator is acomposite of inorganic particles and polymeric binder. The separatorcomposite materials are low in cost and function to provide highperformance separator membrane structures which have excellent adhesionto electrodes and which improve safety by reduction of Li plating. Theirhigh dimensional stability at high temperatures also enhances safety.Furthermore, the membrane materials may be directly coated ontoelectrodes of the battery thereby simplifying fabrication and handlingprocedures. The electrode/membrane assembly exhibits excellent adhesionbetween the layers and does not delaminate from its substrate (currentcollector) even when wound, bent, flexed or otherwise deformed.

In one aspect, separator for an electrochemical cell includes a porouscomposite layer adhered on a porous support, the composite layercomprising electrochemically stable inorganic particles having aparticle size less than 1 μm in an electrochemically stable polymermatrix, said layer having at least a bimodal pore distribution, whereinthe first, smaller sized pores are substantially uniformly distributedin the layer, and one or more larger pore sizes are randomly distributedin the layer, wherein the dimension of the pores are nanoscale

In another aspect of the invention, an electrode/separator assembly foruse in an aelectrochemical cell, comprising a current collector; aporous composite electrode layer adhered to the current collector, saidelectrode layer comprising at least electroactive particles and abinder; and a porous composite separator layer comprising inorganicparticles substantially uniformly distributed in a polymer matrix toform nanopores and having a pore volume fraction of at least 25%,wherein the separator layer is secured to the electrode layer by asolvent weld at the interface between the two layers, said weldcomprising a mixture of the binder and the polymer.

In one embodiment, the separator has a monomodal pore size distributionand the pore size has a value in the range of 5-500 nm.

In one embodiment, the separator has a monomodal pore size distributionand a first smaller pore size is in the range of about 5-100 nm.

In one embodiment, separator has a monomodal pore size distribution anda larger pore size is in the range of about 100-500 nm, or in the rangeof about 100-200 nm.

In one embodiment, the particles are substantially monodisperse and havea particle size in the range of about 10-500 nm, or in the range ofabout 10-50 nm, or in the range of about 10-20 nm.

In one embodiment, the layer has a pore volume fraction of greater than25%.

In one embodiment, the composite layer comprises inorganic particles andpolymer binder in a weight ratio of about 95:5 to about 35:65 inorganicparticles: polymer, or the composite layer comprises inorganic particlesand polymer in a weight ratio of about 65:35 to about 45:55.

In one embodiment, the polymer comprises a polymer which iselectrochemically compatible with Li-ion cells.

In one embodiment, the polymer is selected from the group of latexpolymers and polyvinylidene fluoride-based polymers.

In one embodiment, the inorganic material is selected from the groupconsisting of silica, alumina, natural and synthetic zeolites and otherelectrochemically stable inorganic particles of appropriate particlesize, and is for example, fumed silica.

In one embodiment, the separator layer has a total thickness in therange of about 2 μm to about 40 μm, or a total thickness in the range ofabout 10 μm to about 20 μm.

In one embodiment, an electrode layer is disposed on upper and lowersurfaces of the current collector and a separator electrode is disposedon both electrode layers.

In one embodiment, the separator layer is substantially free of cracksor defects.

In another aspect, a method of preparing a electrode/separator assemblyfor an electrochemical cell includes providing a porous compositeelectrode layer comprising at least electroactive particles and abinder; providing a coating solution, said coating solution comprising apolymer, solvent system for said polymer, and inorganic particlesdispersed in said solvent, wherein said solvent system is selected tohave at least some solubility for the binder of the electrode layer;coating a surface of said electrode layer with a layer of said coatingsolution, wherein the coating solution penetrates a fraction of thethickness of the electrode layer and dissolves a portion of the binder;and removing the solvent from said coating solution layer to deposit aporous separator layer comprising inorganic particles substantiallyuniformly distributed in the polymer and having a pore volume fractionof at least 25% and to form a solvent weld at an interface between saidporous electrode layer and said porous separator layer.

In one embodiment, the method further comprises curing said polymer.

In one embodiment, curing comprises heat treating the assembly.

In one embodiment, the weight ratio of inorganic particles and polymerin the coating solution is about 95:5 to about 35:65.

In one embodiment, the weight ratio of inorganic particles and polymerin the coating solution is about 65:35 to about 45:55.

In one embodiment, the solvent system is a mixture of solvents and thesolvents include a first liquid that is a solvent for the binder and asecond liquid that is a poorer solvent for the binder than the firstliquid and the proportion of first and second liquids is selected tolimit the dissolution of the binder during the coating step.

In one embodiment, the solvent system is a mixture of solvents and thesolvents include a first liquid that is a solvent for the binder and asecond liquid that increases the viscosity of the coating solution andthe proportion of first and second liquids is selected to reduce thepenetration of the coating solution into the thickness of the electrodelayer.

In one embodiment, the solvent system comprises N-methyl pyrrolidone, orthe solvent system comprises a mixture of N-methyl pyrrolidone and adiluting solvent selected from the group consisting of acetone, propylacetate, methyl ethyl ketone and ethyl acetate.

In one embodiment the coating solution penetrates up to 90% of thethickness of the electrode layer, or up to 50% of the thickness of theelectrode layer, or up to 25% of the thickness of the electrode layer,or up to 10% of the thickness of the electrode layer.

In one embodiment, coating is carried out by a technique selected fromthe group consisting of spray coating, doctor blading, slot die coating,gravure coating, ink jet printing, spin coating and screen printing.

In one embodiment, spray coating the surface of said electrode comprisesspray coating a plurality of layers of said coating solution onto saidsurface of said electrode.

In one embodiment, the method further includes drying the coated layerbetween each spray coating step.

In one embodiment, removing said solvent comprises evaporating saidsolvent, or extracting said solvent with a material which is anon-solvent for said polymer.

In another aspect, a method of preparing a defect-free separatormembrane is provided, where defects are defined as discontinuities whichshort the cell or permit unacceptably high leakage currents for anelectrochemical cell, or otherwise reduce performance in cycling andpulsing the cell currents. The method includes providing a coatingsolution, said coating solution comprising a polymer having a meltingtemperature, a solvent for said polymer, and an inorganic materialdispersed in said solvent; providing a support; coating a surface ofsaid support with a plurality of layers of said coating solution withdrying by heat after each deposition, each said layer depositing aportion of the final thickness; and subjecting the membrane with thedesired thickness to a stress-relieving treatment, whereby a porous bodycomprised of said polymer and said inorganic is deposited on saidsurface of said electrode, said body comprising being substantially freeof cracks and other defects.

In one embodiment, the method further removing the solvent from saidlayer prior to the step of stress-relieving treatment.

In one embodiment, the stress-relieving treatment comprises heating thelayer to soften the polymer.

In another aspect a laminate electrochemical cell includes a stack oflayers arranged to provides a positive electrode layer/separatorlayer/negative electrode layer/separator layer repeat unit, where thepositive electrode layer comprises a porous composite positive electrodelayer adhered to both sides of a positive current collector, saidelectrode layer comprising at least electroactive particles and abinder; the negative electrode layer comprises a porous compositenegative electrode layer adhered to a both sides of a negative currentcollector, said electrode layer comprising at least electroactiveparticles and a binder; the separator layer comprises a porous compositeseparator layer comprising inorganic particles substantially uniformlydistributed in a polymer matrix to form nanopores, wherein eachseparator layer is bonded to an adjacent electrode layer through asolvent weld at an interface between the separator layer and theelectrode layer.

In one embodiment, the separator layers comprise about 40-65 wt %polymer.

In one embodiment, the separator layer has a pore volume fraction of atleast 25%.

In one embodiment, the resistance of the cell is greater than 20 MΩ.

In one embodiment, the cell further comprises an electrolyte.

In one embodiment, the separator layer has a total thickness in therange of about 2 μm to about 40 μm.

In one embodiment, the inorganic particles of the separator layer aresubstantially monodisperse and have a particle size has a value in therange of about 10-500 nm, or in the range of about 10-50 nm.

In one embodiment, the separator has a pore size distribution and thepore size has a value in the range of 5-500 nm.

In one embodiment, the separator layer comprises inorganic particles andpolymer in a weight ratio of about 65:35 to about 45:55.

In another aspect, a method of making a laminate electrochemical cellincludes providing stacked cell units comprising a plurality of positiveelectrode layer/separator layer/negative electrode layer/separator layerrepeat units, where the positive electrode layer comprises a porouscomposite positive electrode layer adhered to both sides of a positivecurrent collector, said electrode layer comprising at leastelectroactive particles and a binder; the negative electrode layercomprises a porous composite negative electrode layer adhered to a bothsides of a negative current collector, said electrode layer comprisingat least electroactive particles and a binder; the separator layercomprises a porous composite separator layer comprising inorganicparticles substantially uniformly distributed in a polymer matrix toform nanopores; and applying heat and/or pressure to the stacked cellunits to soften the binder and polymer of the electrode and separatorlayers and fuse adjacent separator and electrode layers to form asolvent weld.

In one embodiment, the separator layer is wetted with a solvent prior toapplication of heat and/or pressure.

BRIEF DESCRIPTION OF THE DRAWING

The invention is described with reference to the figures listed below,which are presented for the purpose of illustration only and are notintended to be limiting of the invention.

FIG. 1 is a schematic illustration of an electrochemical cell includinga porous separator according to one or more embodiments of theinvention.

FIG. 2 is a flow diagram of the spray coating process used in one ormore embodiments of the invention to prepare a separator membrane.

FIGS. 3A, 3B, and 3C are photomicrographs of porous separator coatingsof several thicknesses that have been prepared by single or multiplepass spray coating methods.

FIG. 4 is a photomicrograph of a spray coated separator film accordingto one or more embodiments demonstrating a pore size distribution whichis both nanoscale and bimodal.

FIG. 5 is a plot of % first discharge at different discharge rates(C/10-300 C) for full discharge of a test sample and a comparison sampleusing a pouch cell having either the composite separator coating or afree standing separator film, wherein this separator film is speciallydesigned at high cost for high power, according to one or moreembodiments of the invention.

FIG. 6 is a plot of % third discharge vs. cycle at high charge anddischarge rates for a test sample and a comparison sample using aprismatic cell according to one or more embodiments of the invention.

FIG. 7 is a plot of discharge capacity (mAh) vs. discharge C-rate forpouch test cells and comparison cells having commercially availableseparator membranes.

FIG. 8 is a plot showing pore size distribution for a porous compositemembrane separator according to one or more embodiments.

FIG. 9 is a schematic cross-sectional illustration of a stackedelectrochemical cell according to one or more embodiments of theinvention.

FIG. 10 is a plot of discharge capacity vs. C-rate for prismatic cellsall having composite separators, in which case one composite separatorhas significant cracking in comparison to the others.

FIG. 11 is a plot of % initial discharge capacity vs. cycle forlaminated and unlaminated cells.

DETAILED DESCRIPTION

A porous composite membrane having simplified fabrication, improvedsafety, and enhanced operational performance is described. The porouscomposite membrane can be used as a separator membrane in anelectrochemical device such as a battery, for example, a secondary Liion battery. The separator can be formed with strong adhesion to theelectrode layer, while maintaining the integrity of theelectrode/current collector assembly. This is an attractive feature asmany solvent-applied systems (as will be discussed in greater detailbelow) tend to delaminate the electrode from the underlying currentcollector. In addition, the separator membrane can be prepared over arange of porosity, while providing adequate ionic conductivity andmechanical strength.

Reference is made to FIG. 1, which illustrates an exemplaryelectrochemical cell 10 including a cathode active layer 11, a cathodesubstrate or current collector 12, an anode active layer 13 and an anodesubstrate or current collector 14. The cathode and/or the anode activelayer typically include a porous particulate composite including anelectrode active material, a conductive additive and a polymer binder. Aporous composite separator 15 separates the electrode layers. A liquidelectrolyte permeates the porous separator membrane. The currentcollector is in contact with its respective electrode layer to permitcurrent flow during charge and discharge cycles of the electrochemicalcell. The cells may be stacked or wound together to form a prismatic orspirally wound battery. In such instances, the electrode may be coatedon both sides with an electroactive layer.

As used herein, “cathode” and “positive electrode” are usedinterchangeably. Also as used herein, “anode” and “negative electrode”are used interchangeably.

Also, as used herein, “particle size” refers to the aggregate particlesize. Aggregate particle refers to branched chains of fused primaryparticles. Aggregate particle size refers to the average maximumdimension of the aggregate particles and not the primary particlesmaking up the aggregate particle. Aggregates are further distinguishedfrom agglomerates, which are loose associations of aggregates that canbe readily dispersed.

By “nanoscale,” it is meant less than 500 nm, and preferably less than100 nm.

The cathode layer 11 may be a porous composite particulate layer. Thecathode active material may be a conventional cathode active materialfor a lithium ion secondary battery, such as a lithium-transitionmetal-phosphate compound, LiCoO₂, LiNiO₂ or LiMn₂O₄ and, the like. Thelithium-transition metal-phosphate compound may be optionally doped witha metal, metalloid, or halogen. The positive electroactive material canbe an olivine structure compound LiMPO₄, where M is one or more of V,Cr, Mn, Fe, Co, and Ni, in which the compound is optionally doped at theLi, M or O-sites. Deficiencies at the Li-site are compensated by theaddition of a metal or metalloid, and deficiencies at the O-site arecompensated by the addition of a halogen.

The positive electrode containing the positive electroactive materialhas a specific surface area of the electrode measured using the nitrogenadsorption Brunauer-Emmett-Teller (BET) method after the densificationor calendaring step that is greater than 10 m²/g or greater than 20m²/g. In some embodiments the cathode active material includes a powderor particulates with a specific surface area of greater than 10 m²/g, orgreater than 15 m²/g, or greater than 20 m²/g, or even greater than 30m²/g. A positive electrode can have a thickness of less than 125 μm,e.g., between about 50 μm to 125 μm, or between about 80 μm to 100 μm oneach side of the current collector, and a pore volume fraction betweenabout 40 and 70 vol. %. The active material is typically loaded at about10-20 mg/cm², and typically about 11-15 mg/cm².

The anode layer 13 may also be a porous composite particulate layer. Inone embodiment, the negative active material is a carbonaceous materialor a lithium intercalation compound. An example here would be lithiumtitanate as used in FIGS. 5 and 6. The carbonaceous material may benon-graphitic or graphitic. A graphitized natural or synthetic carboncan serve as the negative active material. Although non-graphitic carbonmaterials or graphite carbon materials may be employed, graphiticmaterials, such as natural graphite, spheroidal natural graphite,mesocarbon microbeads and carbon fibers, such as mesophase carbonfibers, are preferably used. The carbonaceous material has a numericalparticle size (measured by a laser scattering method) that is smallerthan about 25 μm, or smaller than about 15 μm, or smaller than about 10μm, or even less than or equal to about 6 μm.

In some embodiments, the negative active material consists of powder orparticulates with a specific surface area measured using the nitrogenadsorption Brunauer-Emmett-Teller (BET) method to be greater than about2 m²/g, or 4 m²/g, or even about 6 m²/g. The negative electrode can havea thickness of less than 75 μm, e.g., between about 20 μm to 65 μm, orbetween about 40 μm to 55 μm on both sides of the current collector, anda pore volume fraction between about 20 and 40 vol. %. The activematerial is typically loaded at about 5-20 mg/cm², or about 4-5 mg/cm².

The electroactive material, conductive additive and binder are combinedto provide a porous composite electrode layer that permits rapid lithiumdiffusion throughout the layer. The conductive additive such as carbonor a metallic phase is included in order to improve its electrochemicalstability, reversible storage capacity, or rate capability. Exemplaryconductive additives include carbon black, acetylene black, vapor growncarbon fiber (“VGCF”) and fullerenic carbon nanotubes. Conductiveadditives are present in a range of about 1%-5% by weight of the totalsolid composition of the electrode. The binder used in the electrode maybe any suitable binder used as binders for non-aqueous electrolytecells. Exemplary materials include a polyvinylidene fluoride(PVDF)-based polymers, such as poly(vinylidene fluoride) (PVDF) and itsco- and terpolymers with hexafluoroethylene, tetrafluoroethylene,chlorotrifluoroethylene, poly(vinyl fluoride), polytetraethylene (PTFE),ethylene-tetrafluoroethylene copolymers (ETFE), polybutadiene,cyanoethyl cellulose, carboxymethyl cellulose and its blends withstyrene-butadiene rubber, polyacrylonitrile, ethylene propylene dieneterpolymers (EPDM), styrene-butadiene rubbers (SBR), polyimides,ethylene-vinyl acetate copolymers.

The cathode and/or anode electrode layers can be manufactured byapplying a semi-liquid paste containing the appropriate electroactivecompound and conductive additive homogeneously dispersed in a solutionof a polymer binder in an appropriate casting solvent to both sides of acurrent collector foil or grid and drying the applied positive electrodecomposition. A metallic substrate such as aluminum foil or expandedmetal grid is used as the current collector. To improve the adhesion ofthe active layer to the current collector, an adhesion layer, e.g., thincarbon polymer intercoating, may be applied. The dried layers arecalendared to provide layers of uniform thickness and density.

Separator membrane 15 is a porous composite material including inorganicfiller (or ceramic) particles and polymer. The separator is formed froma highly uniform distribution of inorganic filler material and polymer,that is, there is no discernible unevenness in the distribution ofpolymer and an inorganic filler material throughout the membrane. Thereare substantially no regions of the membrane having discernible regionsof predominantly polymer or ceramic material. This highly uniformdistribution is observable even under high magnifications typical of SEMmicroscopy. The separation materials should be electronically insulationwhen used in an electrochemical cell.

A separator membrane for an electrochemical cell may be formed directlyonto an electrode by utilizing a coating solution comprised of apolymer, a solvent system for the polymer and a ceramic materialdispersed in the solvent. Application of the separator membranecomponents from a solution onto the electrode layer provides a durablebond between the two layers. The separator precursor solution is coatedonto a surface of an electrode so as to form a liquid layer. The solventis removed from this layer leaving a porous solid body comprised of thepolymer and the ceramic material. Finally this polymer is cured byheating for a period of time to a temperature greater than the polymermelt temperature (T_(m)) or glass transition temperature (T_(g)). As aresult, the separator membrane is directly bonded to the surface of theelectrode, so that the membrane has unusually good adhesion to theelectrode active layer. This excellent adhesion improves performance byreducing interfacial resistance between the electrodes and the separatormembrane.

Improved adhesion may arise from the solubility of the electrode binderin the solvent system used for putting down the porous separatormembrane. The solvent solubilizes the binder of the underlying electrodelayer so that the binder swells and mixes with the deposited porouscomposite. In other embodiments, the polymer in the porous composite cananneal at elevated temperatures, which can lead to the melting andfusing of the polymer content of the separator membrane layer with theunderlying electrode layer. The adhesion of the separator membrane tothe adjacent porous composite electrode layer in this manner can bereferred to as “solvent welding.” In solvent welding, a solvent commonto the binders in both the electroactive and separator membrane layersdissolve the polymers, causing them to co-mingle in solution. Uponsolvent removal, the polymers are redeposited to bond more effectivelythe particles from both layers. In other embodiments, the bindersexperience melting or softening in a similar temperature regime, so thatthe two polymer binders co-mingle by softening rather than dissolving.

The separator membrane can be applied to any substrate. It can beapplied at the desired total thickness to one electrode, or both theanode and the cathode may be coated with a porous composite layer, ofthe same or different composition and thicknesses. The separatormembrane can have a thickness in the range of 2-40 μm. In thoseinstances where both the cathode and anode are coated with a porouscomposite layer, the amount deposited from each layer can be reduced.For example, where it is desired to form a composite separator of about20 μm, both the cathode and the anode can be coated to form a layerthickness that is substantially half the desired amount. It is desirableto have the separator layer to be as thin as possible, as this increasesionic conductivity and increases capacity in the cell. In order toimprove conductivity, the porosity of the cell is desirably high.However, porosity should not result at the expense of mechanicalstrength. Composites that have uniform, interconnected porosity on ananoscale, e.g., less than about 500 nm, can provide both ionicconductivity and mechanical strength.

In some embodiments the ceramic particles have a specific surface areaof greater than 50-200 m²/g. In one or more embodiments, the ceramicparticles have a substantially uniform particle size, e.g., aremonodisperse, and the particle size is less than 1 μm. In one or moreembodiments, the particle size is in the range of about 10 nm-500 nm, or10 nm-200 nm, or about 10-20 nm.

In one or more embodiments, the organic particles form a continuouslayer in combination with the polymer to provide a monomodal pore sizedistribution. The polymer binds together the free flowing separateparticles of inorganic material and the particles are distributed wellin the polymer. The polymer appears to be mixed intimately with theinorganic component to create a continuous structure. In one or moreembodiments, the polymer forms a substantially continuous coating aroundthe ceramic particles, resulting in a structure which appears to haveone primary building block (no phase or materials separation), ratherthan regions of inorganic particulate and regions of polymer, whenobserved at high resolution, for example with an SEM. The pore size isin the range of about 5 nm-500 nm. The total pore volume is sufficientto provide the desired level of ionic conductivity and is typically atleast 25%, but can be great than 50%, or greater than 75%, and even upto 90% in some instances. Exemplary porosity measurements ofcompositions containing a range of fumed silica (balance PVDF) are foundin Table 1.

TABLE 1 Total % Porosity vs. Composition % Filler (fumed silica) Total %Porosity 35 47 45 56 65 75

In one or more embodiments, the organic particles form a continuouslayer in combination with the polymer to provide an (at least) bimodalnanoscale pore size. In one or more embodiments, the separator membraneincludes a narrow pore size distribution having a value in the range ofabout 5-100 nm, or 5-50 nm, or 5-10 nm, and spanning a range of about5-10 nm. This narrow pore size distribution is believed to arise fromthe substantially uniform packing of substantially monodispersenanoscale ceramic particles. In one or more embodiments, the separatormembrane layer includes a second, larger nanoscale pore size randomlydistributed in the layer and a second smaller nanoscale pore sizesubstantially uniformly distributed in this same layer. The larger poresizes that are larger than the first nanoscale pore size can have avalue of about 100-500 nm or about 100-200 nm, and spanning a range ofabout 5-25 nm.

While not wanting to be bound to any particular mode of operation, it isbelieved that the smaller pores arise from the formation of aggregates,e.g., tetragonal aggregates, in the coating solution slurry formation,which disrupt the otherwise uniform coating deposition of the nanoscaleceramic particles. In one or more embodiments, there may be a pore sizedistribution that is intermediate to the smallest and largest poresizes. By way of example, in the intermediate pore size distribution canbe in the range of about 10-400 nm.

FIG. 4 is a SEM photomicrograph of a porous separator prepared accordingto one or more embodiments of the present invention. Under highmagnification, the composite layer exhibits a pore size distributionbased in a single type of polymer/inorganic primary unit as is seen inthe photomicrograph in FIG. 4, which is believed to be unique. The poresize distribution appears bimodal, with relatively larger pores 400(typically ca. 100 nm-200 nm, but less than one micron) distributedrandomly throughout the layer over a background pore size 410 that is ofa finer nanoporous texture (ca. 10 nm). A highly uniform mixing ofpolymer with ceramic particles is formed, such that distinct regionswhich are only polymer or only ceramic are not found, or are quitescarce. Pore size and pore size distribution may be determined usingconventional methods. By way of example, pore size may be determinedusing thermoporometry by differential scanning calorimeter, mercuryporosimetry, liquid displacement methods and gas sorption techniques.Porosimetry is a technique used to determine as pore diameter, totalpore volume, surface area, and density. The technique involves theintrusion of a non-wetting liquid (often mercury) at high pressure intoa material through the use of a porosimeter. The pore size can bedetermined based on the external pressure needed to force the liquidinto a pore against the opposing force of the liquid's surface tension.This technique is used to determine the pore size of separator membranesin the examples below.

There are a number of materials which may be used in the preparation ofporous separator membranes. The polymer is selected from those polymerswhich are compatible with the chemistry of a particular battery system.The polymer should be electrically insulating, should have lowsolubility in electrolyte solvents and be chemically andelectrochemically stable in the cell. The polymer may be a singlepolymer or a mixture of polymers. Exemplary materials include apolyvinylidene fluoride (PVDF)-based polymers, such as poly(vinylidenefluoride) (PVDF) and its co- and terpolymers with hexafluoroethylene,tetrafluoroethylene, chlorotrifluoroethylene, poly(vinyl fluoride),polytetraethylene (PTFE), ethylene-tetrafluoroethylene copolymers(ETFE), polybutadiene, cyanoethyl cellulose, carboxymethyl cellulose andits blends with styrene-butadiene rubber, polyacrylonitrile, ethylenepropylene diene terpolymers (EPDM), styrene-butadiene rubbers (SBR),polyimides, ethylene-vinyl acetate copolymers. One group of polymershaving utility in lithium and lithium ion battery systems, as well asother battery systems, includes fluorinated polymers and latex polymerssuch as styrene butadiene and other styrene-based polymers. Latexpolymer systems tend to form polymer suspensions and may not besolubilized in the liquid carrier. Polyvinylidene fluoride polymercompositions including polyvinylidene fluoride copolymers andterpolymers are one group of polymers having specific utility. There area variety of such materials known and available in the art, and suchmaterials may comprise essentially homogeneous PVDF as well as blendsand copolymers. One particular material is a PVDF material sold underthe trademark Kureha 7208. Other equivalent and similar materials maylikewise be employed. See, for examples, the materials discussed abovefor the preparation of the anode and cathode active layers.

The inorganic component may be selected from a variety of natural andartificial materials that are compatible with the particular batterysystems and chemistry in which the membranes are to be incorporated.Mixtures of two or more suitable inorganic components are contemplated.The inorganic component may be a ceramic material. One particular groupof ceramic materials comprises silica, with fumed silica being onespecific form of silica which may be employed. Fumed silica is a highsurface area, generally high purity silica material. Fumed silica isgenerally hydrophilic and can be wetted easily by most electrolytesolvents and many polar polymers. A material which has been used in oneor more embodiments has a surface area of approximately 200 m²/g. Theparticles are very small and typically are less than 500 nm in diameter,or less than 200 nm in diameter, and more typically about 10-20 nm. Inone or more embodiments, the ceramic material is fumed silica having anarrow particle size distribution and a substantially spherical shape.Fumed silica can be prepared in a carefully controlled reaction ofsilicon tetrachloride (SiCl₄) that results in a highly controllable andnarrow particle size distribution. In one embodiment, a fumed silicahaving a particle size of about 14 nm may be employed.

Other silicon compounds may be utilized as a ceramic component of themembranes, such as for example, polyhedral oligomeric silesquioxane(POSS), which in the context of this disclosure is considered to be aceramic material. Other ceramic materials include natural and syntheticzeolites, alumina, titania and the like. In addition, otherelectrochemically stable inorganic particles of appropriate size can beused, e.g., MgO, CaCO₃ and other metal carbonates, zirconia, siliconphosphates and silicates. The ceramic materials may be used eithersingly or in combination, with uniform or mixed sizes and shapes aswell.

The proportions of polymer and inorganic materials may vary over arelatively wide range. In some instances, the ratio of ceramic topolymer may range, on a weight basis, from 95:5 to 35:65. In someinstances, the ratio of ceramic to polymer may range, on a weight basis,from 65:35 to 45:55. In one specific instance, the membrane willcomprise, on a weight basis, approximately 65% fumed silica and 35%PVDF. In one or more embodiments, the solids load of the coatingsolution is about 1 wt % to about 20 wt %, or about 3 wt % to about 10wt %.

The presence of a significant amount of organic polymer component isdistinguishable from prior art compositions, which are even morepredominantly inorganic (>90:10) and which typically use significantlylarger particle size ceramic materials. Without being bound to anyparticular mode of operation, it is hypothesized that the polymerorganic provides flexibility and mechanical strength, without impedingthe porosity provided by the packing of the substantially sphericalparticles of the inorganic filler material. Higher polymer levels alsopromote the fusion bonding of adjacent porous layers in anelectrochemical cell prepared using the porous separator membrane.

The solvent system used in the preparation of the coating solution maycomprise any solvent system in which at least one component of thecoating solution is capable of dissolving the polymer component.Suitable second or further components may be used; if not capable ofdissolving the polymer, the additional components are highly misciblewith the first solvent. Preferably, the solvents are relatively easy toremove during subsequent processing steps. One solvent which has beenfound to have utility in connection with PVDF-based membranes includesN-methyl pyrrolidinone (NMP), and the NMP may be blended with anothersolvent such as acetone, ethyl acetate, and propyl acetate for example,to obtain the appropriate slurry rheology. By way of example, solventsof different boiling points may be used to control solvent evaporationrates and thus film stresses which are generated during drying of theliquid slurry.

One specific solvent mixture which was utilized in one implementation ofthe present invention comprised, on a volume basis, a 30:70 NMP/acetonemixture. Others include 30% NMP with 70% of propyl acetate, methyl ethylketone (MEK), or ethyl acetate. The composite slurry is a relativelyhomogeneous suspension which is relatively stable in the absence ofshear.

In one or more embodiments, the solvent system is selected to providerobust adherence of the separator membrane to adjacent electrodelayer(s) without undesirable delamination of the electrode layer fromthe current collector. When the electrode layer is deposited, thesurface of the current collector may be treated to promote electrodeadhesion. In addition, the polymer binder promotes adhesion of theelectrode particles to the current collector surface. However, if thesolvating properties of the solvent system used to cast the separatormembrane are too strong or the permeability of the solvent system intothe electrode layer is too high, it may completely or significantlydissolve the binder of the electrode layer and thereby delaminate theelectrode layer from the current collector. The effects of delaminationcan be quite dramatic and it can render the electrode/separator membraneassembly unusable.

Thus, according to one or more embodiments, the solvent system isselected to provide limited solubility of the binder in the electrodelayer. This may be accomplished by appropriate selection of the polymerand solvent system in the casting solution for the separator membrane sothat the solvent system has good solubility for the separator polymer,but lesser solubility for the binder of the electrode layer. In one ormore embodiments this may be achieved by providing a solvent system thatlimits the amount of solvent present that would solubilize the electrodebinder. By way of example, the solvent is blended with a second solventhaving lower solubility for the binder. In one or more embodiments, lessthan 50 vol %, or less than 30 vol %, of the solvent system is a bindersoluble solvent.

In other embodiments, the same polymer is used for the electrode and theseparator layers. That means that the solvent has the same solubilizingeffect on both materials. The solvent system can be adjusted in otherways to prevent delamination of the electrode layer from the electrode.In other embodiments, the viscosity of the solvent system is adjusted toprevent or reduce the level of penetration of the casting solution intothe electrode layer. In one or more embodiments, the casting solutionremains at the interface with electrode layer and does not penetratesubstantially into the electrode layer. By way of example, it does notpenetrate more than 90%, or more than 75%, or more than 50% or more than25% or more than 10% of the thickness of the electrode layer.

Methods of controlling solution viscosity (and thereby solutionpenetration) include controlling the solids content of the coatingsolution. When working with a comma coater, a type of roll coating, lowsolids content coating solutions can lead to delamination. By increasingthe percent solids, and thus the viscosity, delamination can beprevented. For an exemplary fumed silica/PVDF/NMP/acetone system asdescribed herein, 5.5% solids lead to delamination, 8% solids had lessdelamination, whereas 9% solids had no delamination.

The viscosity of the casting solution can also be adjusted by selectionof solvents of differing viscosities.

When working with a spray coating system it may not be possible toincrease viscosity to a level that would prevent penetration since theability to spray a quality mist is related to the viscosity. Onesolution is to reduce the amount of liquid deposited in any given time,since for a given slurry formulation the more liquid that is deposited,the more likely it is to cause delamination. To address delamination ina spray coating system, the number of passes between drying steps isadjusted. In one or more embodiments, multipass deposition of thinlayers of the coating solution is employed to reduce delamination.

The inorganic material and polymer are combined in the solvent system toform a uniform distribution of inorganic particles in the dissolvedpolymer/solvent system. The highly uniform distribution of polymer andinorganic material in the coating solution provides a highly uniformdistribution of polymer and inorganic materials in the resultantmembrane. By blending a poorer solvent into the strong solvent in thecoating solution, a suspension of polymer and inorganic filler iscreated. This suspension helps assure an intimate mixture of the twosolids and prevents particulate separation/segregation during the dryingstep. FIG. 4 is a scanning electron microphotograph (SEM) of a separatoraccording to one or more embodiments that illustrates the uniformdistribution of organic polymer and particulate inorganic components.

A coating method is described with reference to FIG. 2. In step 200, thecoating solution is prepared including a solvent, solvent-soluble orsolvent-miscible polymer and inorganic particles. In one or moreembodiments, the polymer, liquid solvents and inorganic ingredients aremixed under low shear for an initial period until ingredients are fullywetted and/or dissolved. In a preferred method the polymer and inorganicare first mixed in NMP so that a high level of dispersion is achieved.Next, the second solvent is added, and this mixture can then besubjected to a high shear mixture until a desired rheology is obtained.A desirable slurry does not contain large agglomerates and does notquickly phase segregate to separate regions of polymer and inorganicmaterials upon standing but instead remains well dispersed. Withoutbeing bound by any mode or theory of operation, it is believed that thesolution rheology provides an indication of distribution of particlesizes and agglomeration behavior as well as total particleconcentrations. More complex and asymmetric shapes and a larger numberof particles tend to increase the viscosity of a solution. Such slurryproperties may play a role in the final structure of the layer.

The coating solution is then coated onto at least one surface of anelectrode material, as is indicated in step 220. The thickness of thelayer coated onto the electrode will depend upon the particularcomposition of the coating solution and the final thickness desired inthe electrochemical cell. Other coating techniques may be employedaccording to one or more embodiments of the invention, so long as theyare susceptible to depositing a composition including a mixed ceramicand particle composition. Exemplary techniques includes doctor blading,roll coating, slot die coating, ink jet printing, spin coating, gravurecoating and screen printing, or other coating methods. Coating istypically carried out under conditions that provide for solvent weldingbetween the separator membrane layer and the adjacent electrode layer.

In one or more embodiments, coating may be accomplished by spraying oneor more coats of the applicator coating solution thereonto. By way ofexample, the separator layer may be applied in about 3 to 5 coatingsteps, each coating step applying about ⅓ to ⅕ of the total separatorlayer thickness. As noted above, multipass deposition reduces solventpenetration into the electrode porous layer and can help reducedelamination. It has been surprisingly found that the application of theseparator layer in multiple steps significantly reduces the number ofdefects formed in the final layer. Defects are defined as large poreshaving dimensions greater than one micron, or cracks in the film. Thedepositions steps need not apply layers of similar thickness. Thus, afirst coating step can deposit a layer of a first thickness and a secondstep can deposit a layer of a second, different thickness.

Following the coating, step 230 illustrates that the solvent is removedfrom the coating mixture to leave a solid porous body of polymer/ceramicparticles on the electrode. The solvent may be removed by evaporation,and this evaporation may be fostered by use of heating and/or lowpressure conditions. In some instances, the solvent may be extracted bythe use of an extraction solvent which is a non-solvent for the polymer.Such techniques are known in the art. In one or more embodiments, thesolvent optionally may be removed after each spray coating step, so thatmultiple solvent removal steps may be conducted when multiple spraycoating steps are used.

In one or more embodiments, the polymer is a thermoplastic and has aglass transition temperature (T_(g)) and may or may not have a melttemperature (T_(m)). In one or more embodiments, after coating a coatingonto the support, the layer is subjected to a treatment selected toreduce the stress in the layer by curing the layer. The polymers may becured by treatment above their glass transition or melting temperatureso as to modify or enhance its physical properties (step 240). Curingmay be accomplished by heating, as is known in the art. The drying stepand the curing step may or may not be carried out in serial steps. Inthe case of thermoplastic polymers, such as PVDF, curing is accomplishedby heating the composite beyond the host polymer Tm and then allowing itto cool down. In other embodiments, the layer is heated at or above theglass transition temperature of the polymer binder.

It is believed that the multistep coating approach leads to fewer largecracks in the separator film. While not being bound by any particularmode or theory of operation, the second coating may fill the crevicescreated in the initial coating to heal any defects of cracks. Theadvantages of the multistep coating method are illustrated by comparisonof fumed silica/PVDF coatings that have been prepared using a single vs.multiple coatings. Photomicrographs of porous separator coatings ofvarying thicknesses that have been prepared by single or multiple passspray coating methods are shown (FIGS. 3A, 3B, 3C). The layers weredried using the same heating/curing method of drying at 80° C. undervacuum followed by curing at 200° C. for 15 min. The film shown in FIG.3A is 5 μm thick and deposited in a single step; the film shown in FIG.3B is 17 μm thick and deposited in three steps; and the film shown inFIG. 3C is 13 μm thick and deposited in a single step Only, the film inFIG. 3B that was deposited using three passes is without cracks, evenunder high magnification. Cracking has been shown to reduce theperformance of the electrochemical cell. See, Example 7.

The result of the foregoing process is the deposition onto an electrode(or other suitable substrate) of a layer of separator layer comprised ofpolymer and ceramic particulate material that are intimately combinedand nanoporous. The process can be used to apply a porous separatormembrane onto a supporting substrate such as an electrode. Thesemembrane coatings have been found to be durable and highly adherent. Themembrane coated electrode may then be incorporated into battery cells,and the cell may include coatings on either or both of the anode andcathode electrodes. The electrode can be processed into a battery, e.g.,by assembly the current collector, positive electrode, separatormembrane, negative electrode and current collector layers into alaminate structure and then bending or rolling the laminate structureinto the appropriate form. In one or more embodiments, a nonaqueouselectrolyte is used and includes an appropriate lithium salt dissolvedin a nonaqueous solvent. The electrolyte may be infused into a porousseparator that spaces apart the positive and negative electrodes. In oneor more embodiments, a microporous electronically insulating separatoris used.

Numerous organic solvents have been proposed as the components of Li-ionbattery electrolytes, notably a family of cyclic carbonate esters suchas ethylene carbonate, propylene carbonate, butylene carbonate, andtheir chlorinated or fluorinated derivatives, and a family of acyclicdialkyl carbonate esters, such as dimethyl carbonate, diethyl carbonate,ethylmethyl carbonate, dipropyl carbonate, methyl propyl carbonate,ethyl propyl carbonate, dibutyl carbonate, butylmethyl carbonate,butylethyl carbonate and butylpropyl carbonate. Other solvents proposedas components of Li-ion battery electrolyte solutions include γ-BL,dimethoxyethane, tetrahydrofuran, 2-methyl tetrahydrofuran,1,3-dioxolane, 4-methyl-1,3-dioxolane, diethyl ether, sulfolane,methylsulfolane, acetonitrile, propiononitrile, ethyl acetate, methylpropionate, ethyl propionate and the like. These nonaqueous solvents aretypically used as multicomponent mixtures.

A solid or gel electrolyte may also be employed. The electrolyte may bean inorganic solid electrolyte, e.g., LiN or LiI, or a high molecularweight solid electrolyte, such as a gel, provided that the materialsexhibit lithium conductivity. Exemplary high molecular weight compoundsinclude poly(ethylene oxide), poly(methacrylate) ester based compounds,or an acrylate-based polymer, and the like.

As the lithium salt, at least one compound from among LiClO₄, LiPF₆,LiBF₄, LiSO₃CF₃, LiN(SO₂CF₃)₂, LiN(SO₂CF₂CF₃)₂ and the like are used.The lithium salt is at a concentration from 0.5 to 1.5 M, or about 1.3M.

In other embodiments, the process can be used to apply a porousseparator membrane onto a sacrificial support, such as a polymer film.The resultant membrane can be transferred to an electrode or otherelement. The porous composite layer is not required to be formeddirectly onto the electrode, but can be formed on other surfaces. Forexample, the separator can also be prepared on a sacrificial support,e.g., a polymer sheet, which is used to transfer the separator onto anelectrode. This transfer is done in a manner which insures excellentadhesion to the electrode substrate. Additional manufacturing steps canbe avoided by applying the separator directly on the electrode surface.

In one or more embodiments, one or more cell assemblies can be combinedinto an integral body that has high degree of connectivity and lowresistance. It has been surprisingly discovered that stacked electrodelayers can be laminated under heat and pressure without significant lossof porosity, cracking or other defect formations. Conventionalunderstanding would predict that the forces needed to fuse or laminatethe electrode layers would degrade the porous layers leading to shortcircuiting and reduced conductivity (higher resistance).

Applicants have surprisingly discovered that robust laminated cells canbe obtained without such detrimental effects. While not being bound byany particular mode or theory of operation, a higher polymer content inthe porous separator may provide sufficient material resilience to allowthe stacked cells to be laminated without cracking or significantdensification. In one or more embodiments, the separator layer mayinclude about 40-65 wt % polymer.

To prepare a laminate electro chemical cell, electrode-coated currentcollectors and separator membranes can then be stacked to provide astacked assembly 90 as illustrated in FIG. 9, in which like elements aresimilarly numbered. Thus a current collector 14 having anode layers 13on both sides can have a separator layer 15 deposited on one side asdescribed herein. Current collector 12 having cathode layers 11 on bothsides can also have a separator layer 15 deposited on one side asdescribed herein. The stacked assembly may be reassembled using variouscomponents. The stack may include a cathode/separator membrane assembly,which can then be stacked with anode layers to form the stackedassembly. In other embodiments, an anode/separator assembly is combinedwith cathode layers to form the stacked assembly. In still otherembodiments, cathode/separator membrane assemblies and anode/separatorassemblies are used. In this case, the separator membrane layerthickness is adjusted (thinner) to accommodate the separator membranefrom both the anode and cathode assemblies. Any number of cathode andanode layers may be included in the stack, although they are typicallymatched (or may contain an extra of either of the cathode or anodelayer). In one or more embodiments, at least 2, at least 5, at least 10or at least 20 cathode/separator/anode separator repeat units are usedin the stacked cell.

The stacked assembly can be heated at or above the T_(g) or at or abovethe T_(m) of the binder and polymer of the assembly under pressure,e.g., in the direction shown by the arrows in FIG. 9. Upon cooling, theassembly is robustly laminated and has formed a single continuousmonolithic cell which demonstrates significantly reduced resistivity dueto the solvent welding and fusion between the composite separator layersaccomplished by the laminating process. In one or more embodiments, asmall amount of solvent is sprayed onto the porous separator surfacesprior to application of pressure and temperature. The additional solventcan help in the solvation and softening of the polymer/binder. Thelamination is considered to be effective when peeling laminatedelectrodes apart reveals the metal foils, which means that the weakestlink is the electrode/current collector—not the separator/separatorinterface.

TABLE 2A Exemplary Laminating Conditions Porous separator composition(Ratio is Temperature Pressure Time fumed silica:PVDF) (° C.) (psi)(min) Dry 65:35 160 500 3 NMP Wetted 65:35 140 500 3 Dry 45:55 140 500 2

The laminated electrodes are electronically robust and no electricalshort is formed upon lamination, as determined by a high resistancereading, e.g., >20 MΩ, between the laminated anode and cathode with amultimeter. The pores of both the porous separator and the porouselectrode substrate also desirably retain their shape and or size whenpressure/temperature are applied. Maintenance is estimated by measuringthe thickness of the electrode stack before and after laminating. In oneor more embodiments, the reduction in thickness (reduction in porosity)is less than about 10%. The reduction in thickness and resistancereading for exemplary laminated cells are shown in Table 2B.

TABLE 2B Thickness and Resistance Measurements for Exemplary LaminatedCells % loss NCS type Cell no. Lamination condition in thicknessMultimeter 65:35 1 Temp: 160° C. Press: 8.13 >20 MΩ 2 500 psi 8.22 6.4kΩ 3 Time: 5 min 7.86 >20 MΩ 45:55 1 Temp: 140° C. 5.50 >20 MΩ 2 Press:500 psi 5.39 >20 MΩ 3 Time: 3 min 5.61 >20 MΩ

Table 2B illustrates cell properties for two different laminated cellshaving different porous separator compositions (65 wt % fumed silica vs.45 wt % fumed silica) and obtained under different laminatingconditions. As can be seen, the % loss in thickness (which correlates todensification) is very low. In addition, all of the cells had at least20 megaohm resistance, indicating that the cells integrity is maintainedand no short circuit has occurred.

FIG. 11 is plot of % initial discharge capacity vs. cycle number oflaminated and unlaminated cells. Both cells include multiple porousseparators having 65 wt % fumed silica and 35 wt % PVDF in a stackedelectrode. Both curves show very little capacity loss over many cycles.In fact, the laminated cell appeared to show a slight advantage inretaining discharge capacity (the upticks in the curve can bedisregarded as artifacts), indicating that the electrochemical andmechanical integrity of the cell is maintained after laminating.

A porous separator is formed having desirable mechanical,electrochemical and safety features. By way of example, aninorganic/organic composite separator according to one or moreembodiments that is aggressively cycled (+10 C/−10 C) shows little Liplating on the anode of the cycled cell. In comparison, a cell madeusing standard separator membrane shows significant Li plating on theanode under similar conditions. Without being bound by any theory ofoperation, it is hypothesized that the unique nano-structure of a porouscomposite separator according to one or more embodiments positivelyinfluences charge distribution and mass transport thereby reducinglithium plating.

The specific properties of the membrane in terms of composition,thickness, physical properties and the like will depend upon particularbattery systems in which the membranes are to be incorporated. Furtherillustration is provided in the following examples, which are presentedfor the purpose of illustration only and are not intended to be limitingof the invention.

EXAMPLE 1 Preparation of a Porous Separator

Membranes for lithium-ion cells were prepared from a 65:35 mixture offumed silica and PVDF. Membranes were prepared from Kureha 7208 PVDF NMPsolution and fumed silica with a surface area of approximately 200 m²/g.The ratio of silica to polymer was 65:35 on a weight basis, and a 7%loading of these solids was slurried in a 30:70 (volume/volume) mixtureof NMP and acetone. This slurry was prepared by first thoroughly mixingthe silica into the PVDF/NMP solution using an orbital mixer at a lowspeed until highly dispersed, and then slowly adding the acetone to thisdispersion mixing first at low speed and then at very high speeds untila stable suspension is formed. This is then loaded into a HVLP spraygun.

This coating composition was applied to either a body of anode orcathode material intended for use in a lithium-ion cell. The drythickness of the application was approximately 20 microns and it wasapplied in 3 to 5 separate coats, with drying at 80° C. in vacuumbetween each coating. For example, to create a 20 μm separator usingthree separate coating steps, each having an equal dry thicknesses ofapproximately 7 microns, the electrodes were sprayed three times so thata wet thickness of ˜100 μm was deposited. (Evaporation of the solventsduring the drying process reduced the thickness of each layer from 100μm to ˜7 μm.)

After each coating, the electrode was vacuum dried at 80° C. for 1 hourand then finally cured at 200° C. for 15-60 minutes in air at ambientpressure after the last coating is applied. The resultingelectrode/separator membrane structures were employed in a variety ofcell architectures including coin cells, pouch cells, and stackedprismatic cells.

These coated electrodes were found to function very well. In particular,350 mAh prismatic cells incorporating the foregoing separator were shownto function very well in both limited cycle life performance testscompared to cells using conventional membrane separators Cells utilizingthe foregoing separators show comparable cycle life relative toconventional membrane separator cells. In smaller capacity laboratorycells incorporating the foregoing separator it was observed thatsubstantially higher power could be obtained than in similar cells madeincorporating a conventional membrane separator.

EXAMPLE 2 Measurement of Leakage Current

Leakage current is a figure of merit in predicting cell shelf-life.

A porous membrane prepared substantially as described in Example 1 wasprepared with the following modifications. The silica separator was laiddown in either one or two passes onto a lithium iron phosphate basedcathode. In the former, the process protocol is to spray, dry, and thencure the layer. If the latter, then the protocol is to follow thespray/dry/spray/dry/cure technique.

Cells were prepared with Li metal anodes in a single layer pouch formatby placing the coated cathode made of a lithium iron phosphate material(LFP) directly adjacent to a counter electrode of Li metal in a pouchcontainer that is sealed on three sides, filling the cell withelectrolyte and then sealing the fourth side so that the interior istotally isolated from the external environment. The cells were cycledthree times (+C/2, −C/5) before being charged to 3.0V and left at opencircuit. The current was monitored for three days and then extrapolatedto full discharge to give the values seen here. Table 3 shows theleakage current measured for a variety of cell types in which the totalseparator thickness and number of coatings were varied. The averageleakage current (and resultant time to discharge) were lower (andlonger) for cells in which the porous silica layer was deposited in twosteps. The leakage current for sprayed-on silica membranes werecomparable to comparison cells made with commercially available Celgard2325 and Gore Excellerator separators. While comparison cellsdemonstrated longer time to discharge than any of the test cells 2A-2D,test cells 2A and 2B demonstrated lower average leakage currents (0.97μA vs. 1.09-1.26 μA).

TABLE 3 Leakage Current for a variety of separator types andmanufacturers in LFP half cells and LFP/LTO cells Number of Average Timeto Total applications Leakage discharge thickness to achieve Currentcell, Cell (+) (−) Separator (μ) thickness (μA) (hours) 2A LFP LiAldrich Si-PVDF (65:35) 34 2 0.93 4334 2B LFP Li Aldrich Si-PVDF (65:35)41 2 1.02 3733 Avg. 0.97 4033 2C LFP Li Aldrich Si-PVDF (65/35) 22 137.46 122 2D LFP Li Aldrich Si-PVDF (65/35) 27 1 16.87 279 Avg. LFP Li27.17 201 Avg. 384.94 1780 1.26 3981 Comparison 2E LFP Li Goreexcellerator 30 NA 1.26 3981 2F LFP Li Gore excellerator 30 NA 1.03 4842Avg. 1.14 4412 2G LFP Li Celgard 2325 25 NA 1.13 4354 2H LFP Li Celgard2325 25 NA 1.05 4767 Avg. 1.09 4560

EXAMPLE 3 Evaluation of Cell Life Cycle

Cell power is an important figure of merit. The impact of separatorchoice on cell power is estimated by comparing cells which are the samewith exception of separator—this has been done using a pouch cellincluding a cathode comprising a lithium iron phosphate containingelectroactive material (“M1”) and an anode comprising a lithium titanate(“LTO”).

A porous membrane prepared substantially as described in Example 1 wasprepared with the following modifications. The silica separator wasprepared as follows. electrodes (2 cm×2 cm) of M1 and LTO were eachcoated with about 2-5 coats of silica separator as previously described.These electrodes were then placed in a polymeric pouch, flooded withmixed carbonate/LiPF₆ electrolyte appropriate for a lithium ion batteryand sealed to the outside environment. Similarly, 2 cm×2 cm electrodesof LTO and M1 were prepared and sealed into a similar pouch separated bya polyolefin membrane. In all respects, save the separator, these twocells are identical.

Cells were prepared with LTO anodes in a single layer pouch formatsubstantially as described in Example 2.

Comparison cells were prepared as described above, except that theseparator was a porous polyfluorinated polymer membrane available fromGore, Inc. under the trade name Gore Excellerator having a thickness ofapproximately 23 μm

The cells were cycled through charge/discharge cycles at increasingcharge rates (1/10 C, 1 C, 3 C, 5.7, 10 C, 1/10 C, 5 C, 100 C, 200 C and300 C). Cycling at each charge rate was carried out for about 3-10cycles. The cell performance was monitored by plotting the percent offirst discharge at each cycle for both the test cell and the comparisoncell. As is shown in FIG. 5, the test cell using the polymer ceramichaving a nanocomposite separator performed as well as the comparisoncell having a Gore Excellerator separator up to 50 C and then exceededcell performance of conventional standard separator at charge ratesabove 50 C.

EXAMPLE 4 Evaluation of Cell Life Cycle

Cell life cycle performance for a prismatic cell was investigated. Powervalue is estimated by comparing cells which are the same with exceptionof separator—this has been done with prismatic cells.

Cells were prepared with LTO anodes in a prismatic cell format byalternately stacking about 13 anodes with about 12 cathodes. Those cellsconstructed using the nanocomposite separator were made with theelectrodes in direct contact with each other and a layer of polyolefinmembrane wrapped around the entire stack before being impregnated withelectrolyte and vacuum sealed in a pouch and run through a formationprocess. After the formation cycles, the pouch was vented, resealed andtested. In the case of the polyolefin separated cell, the anode andcathode was separated from one another by accordion folding the membranearound the two electrodes. In all other respects the two cells wereprepared identically.

Comparison cells were prepared as described above, except that theseparator was a porous polyfluorinated polymer membrane available fromGore, Inc. under the trade name Gore Excellerator having a thickness ofapproximately 23 μm.

The cells were cycled through charge/discharge cycles at +3 C/−8.7 C forup to 4000 cycles and cell performance was monitored by plotting thepercent of third discharge at each cycle for both the test cell and thecomparison cell. As is shown in FIG. 6, the test cell performed as wellas the comparison cell.

EXAMPLE 5 Evaluation of Cell Life Cycle

Power is estimated by comparing the capacity vs. rate for cells whichare the same with exception of separator—this has been done with pouchcells having a graphite anode.

A porous membrane prepared substantially as described in Example 1 wasprepared using the method in which final thickness is achieved by threeseparate spray coat/dry steps.

Cells were prepared with graphite anodes in a single layer pouch formatsubstantially as described in Example 2 with the exception that agraphite composite anode was used instead of an LTO composite anode.

Comparison pouch cells (Comparison Cell #1) were prepared as describedabove, except that the separator was a porous polymer membrane availablefrom Celgard Corporation under the trademark Celgard 2320. The separatorwas made from porous polyolefins and had a thickness of approximately 20microns. Another comparison cell (Comparison Cell #2) was prepared usinga porous separator available from Degussa, Inc. under the trade nameSeparion S240P25 having a thickness of approximately 25 μm.

The cells were cycled through charge/discharge cycles at increasingdischarge rates (0.1 C-500 C) and each charge rate was carried out forabout 3-10 cycles. The cell performance was monitored by plottingdischarge capacity (mAh), discharge C-rate for both the test cell andthe comparison cells. As is shown in FIG. 7, the test cell performed aswell as the comparison cells over a range of discharge rates.

EXAMPLE 6 Determination of Pore Size Distribution

Pore size distribution was obtained using mercury intrusion analysis.The samples were uncoated cathode, cathode coated with separator of65-wt % fumed silica, and cathode coated with 45-wt % fumed silica. Thematerial for analysis was obtained by removing it from the currentcollector by carefully bending the samples over a razor blade andpulling the blade along the back side of the samples while maintainingtension. This motion results in dislodging of the coating on the frontof the sample and it will come off of the current collector withoutcontacting that side. The dislodged samples were relatively largeflakes—not powder. The pore size distribution was determined usingstandard procedures with these flakes with an AutoPore MercuryPorosimeter.

Results are shown in FIG. 8. Because the separator samples also includedcathode material, the pore size distribution for the cathode subtractedout from the separator samples, leaving, in the case of the 45-wt %fumed silica sample, with negative peaks. Pore sizes of 25 nm, 50 nm and100 nm are clearly observed for separator of 65-wt % fumed silicasample. Pore sizes of 40 nm and 120 nm are clearly observed forseparator of 45-wt % fumed silica sample. Thus, bimodal and multimodalpore size distribution is observed.

EXAMPLE 7 Effect of Separator Cracks on Discharge Capacity

Cells were prepared using a fumed silica/PVDF system with differentcompositions and properties as set forth in Table 4 and are generallyprepared as described above.

TABLE 4 Composition of Test Cells Fumed Silica PVDF (wt Location ofseparator Cell Name %) %) layer comments NCS Build 8 65 35 20 μmdeposited on cathode/0 on anode NCS Build 9 45 55 20 μm deposited oncathode/0 on anode NCS Build 45 55 20 μm deposited on Cracks 10cathode/0 on anode intentionally introduced Wide Standard polyolefin

In this table cells having a porous separator of varying compositions,without cracks and one which was used with deliberate cracks (build 10in black). As well, the performance of a cell with a standard polyolefinseparator was evaluated—this separator is made by Wide and the cell issimply called “Wide”.

FIG. 10 is a the plot of capacity vs. C-Rate for the cells listed abovein Table 4. An initial observation is that cells having high porosityseparators (Build 8 and 9) have capacities that are comparable tocommercially available porous polyolefin separators. NCS Build 10 wasprepared from a high porosity separator that was deliberately treated toinduce cracking. The reduced performance clearly shows that eliminationof cracks is critical to high performance.

EXAMPLE 8 Conductivity Measurements

Conductivity was measured using Li-Ion cells with the describedseparators and composite electrodes having PVDF binders as shown inTable 5. The anode was an MCMB graphitic carbon and the cathode wasbased on lithium iron phosphate. Electrolyte was a mixtures ofcarbonates and LiPF₆ salt. Cells impedance was measured when the cellvoltage was >2.8Volts open circuit. The complex impedance was measuredover the frequency range of 0.01 Hz to 100,000 Hz using a SolartronFrequency generator and analyzer. The sinusoidal voltage was 5 mV peakto peak. The resistance is that value when capacitance is at a minimumat low frequencies—which is where the plot crosses the x-axis in a plotof imaginary vs. real impedance. These resistance values are convertedinto bulk conductibility based on the electrode cross-section andseparator thickness.

TABLE 5 Conductivity Measurements of Exemplary Li-Cells. CONDUCTIVITYSAMPLE (mS/cm) at 20μ Commercial Polyolefin Separator of 20 0.55 micronsthickness and 45% porosity NCS 65:35 0.50 NCS 45:55 0.30

These results indicate that the conductivity of the porous separatorwill vary with composition. In the present example, porous separatorshaving a higher fumed silica content (65-wt %) had conductivitymeasurements that were comparable to commercial polyolefin separators.Reducing the inorganic particle content of the separator to 45-wt %resulted in a reduction in cell conductivity.

EXAMPLE 9 Adhesion Testing

The relative adhesive strength of the porous separator to the electrodelayer and the electrode layer are determined according to the followingtest.

TAPE TEST: Ordinary scotch tape is placed on the top of the substrateand secured by pressing with your finger—this turns the tape fromtranslucent to clear. One edge is kept free of the substrate. This freeedge is pulled away from the substrate with a fast motion.

PASS SEPARATOR ADHESION: The separator/electrode assembly is removedfrom the film substrate—leaving behind the metallic film substrate(current collector). This occurs because the separator/electrodeinterface is stronger than that of the electrode/current collector. Thisis the typical result.

FAIL SEPARATOR ADHESION: The electrode is revealed as dark region andthe tape has the white deposit (separator is white). This has not yetoccurred for any of the porous separator/electrode assemblies tested,indicating that there is a secure and robust bond between the porousseparator and the porous electrode layer.

MANDREL TEST: A mandrel of 2 mm diameter is suspended at its ends on twomounting bars so that the majority of the center section is notconstrained by the support structure. The film of appropriate length(this is approximately twice the length of the distance from the bar tothe table on which it stands) is draped over the bar. Next it is rolledfrom one end to the other ten times with a force which pulls the filmtightly across the bar—this is done by hand—the two ends are held in twohands. After ten pulls back/forth the test is complete. FAIL: particlesof coating freely fall off the electrode PASS: Particles of film havenot freely fallen from the electrode, and so it is next examined underlight microscope at about 100× and there is not significant cracking.This experiment is used to test the adhesion of a film to asubstrate—most notably it is used to test if the active material is ableto maintain adhesion to the current collector in both extension andcompression similar to what is encountered in a wound cylindrical cell

The foregoing illustrates one specific embodiment of this invention.Other modifications and variations of the invention will be readilyapparent to those of skill in the art in view of the teaching presentedherein. The foregoing is intended as an illustration, but not alimitation, upon the practice of the invention. It is the followingclaims, including all equivalents, which define the scope of theinvention.

The invention claimed is:
 1. A separator for an electrochemical cell,comprising: a porous composite layer adhered on a porous support, thecomposite layer comprising: electrochemically stable inorganic particleshaving a particle size less than 1 μm in an electrochemically stablepolymer matrix, said composite layer having a multimodal poredistribution comprising a smaller pore size, an intermediate pore size,and a larger pore size, wherein the smaller pore size is substantiallyuniformly distributed in the composite layer and the larger pore size israndomly distributed in the composite layer, wherein dimensions of thepores are nanoscale.
 2. The separator of claim 1, wherein the smallerpore size is in a range of about 5-100 nm.
 3. The separator of claim 1,wherein the larger pore size is in a range of about 100-500 nm.
 4. Theseparator of claim 1, wherein the larger pore size is in a range ofabout 100-200 nm.
 5. The separator of claim 1, wherein the particles aresubstantially monodisperse and the particle size is in a range of about10-500 nm.
 6. The separator of claim 1, wherein the composite layer hasa pore volume fraction of greater than 25%.
 7. The separator of claim 1,wherein the composite layer comprises the inorganic particles and apolymer binder in a weight ratio of about 95:5 to about 35:65 inorganicparticles: polymer.
 8. The separator of claim 1, wherein theintermediate pore size is in a range of about 10-400 nm.
 9. Theseparator of claim 1, wherein the polymer matrix comprises a polymerwhich is electrochemically compatible with Li-ion cells.
 10. Theseparator of claim 1, wherein the polymer matrix is selected from thegroup of latex polymers and polyvinylidene fluoride-based polymers. 11.The separator of claim 1, wherein said inorganic particles are selectedfrom the group consisting of silica, alumina, natural and syntheticzeolites, and other electrochemically stable inorganic particles ofappropriate particle size.
 12. The separator of claim 11, wherein saidsilica comprises fumed silica.
 13. The separator of claim 1, wherein thelayer has a total thickness in the range of about 2-40 μm.
 14. Theseparator of claim 1, wherein the support comprises an electrode. 15.The separator of claim 1, wherein the support comprises a polymer sheet.16. The separator of claim 1, wherein the composite layer issubstantially free of cracks or defects.
 17. A separator for anelectrochemical battery cell, comprising: a porous composite layeradhered on a porous support, the porous composite layer comprisingelectrochemically stable inorganic particles having a particle size in arange of about 10-20 nm in an electrochemically stable polymer matrix;wherein the electrochemically stable inorganic particles aresubstantially monodisperse; wherein the porous composite layer has amultimodal nanoscale pore size distribution comprising a smaller poresize, an intermediate pore size, and a larger pore size, the smallerpore size being substantially uniformly distributed in the porouscomposite layer and the larger pore size being randomly distributed inthe porous composite layer; and wherein the intermediate pore size is ina range of about 10-400 nm.
 18. The separator of claim 17, wherein theporous composite layer comprises the electrochemically stable inorganicparticles and polymer in a weight ratio of about 65:35 to about 45:55.19. A separator for an electrochemical battery cell, comprising: aporous composite layer adhered on a porous support, the porous compositelayer comprising electrochemically stable inorganic particles having aparticle size in a range of about 10-50 nm in an electrochemicallystable polymer matrix; wherein the porous composite layer has amultimodal nanoscale pore size distribution comprising a smaller poresize, an intermediate pore size, and a larger pore size, the smallerpore size being substantially uniformly distributed in the porouscomposite layer and the larger pore size being randomly distributed inthe porous composite layer; and wherein the smaller pore size is in arange of about 5-100 nm, the intermediate pore size is in a range ofabout 10-400 nm, and the larger pore size is in a range of about 100-500nm.
 20. The separator of claim 19, wherein the porous composite layerhas a total thickness in a range of about 10-20 μm.